MechChem Africa November 2019

The KNeW process: a profitable way of recovering salinated wastewater

Trailblazer Technologies’ John Bewsey is pioneering cost effective and potentially profitable ways of processing neutralised acid mine drainage (AMD) water, industrial wastewater and even underground ‘brakwater’. In this article, he describes his KNeW (Potassium Nitrate exWaste) process, which relies on scalable ion exchange CSTR batteries with some value adding regeneration and post regeneration twists to transform dissolved contaminants into valuable fertilisers and industrial salts.

A cid mine drainage (AMD) is an expensive problem and one that remains largely unresolved. Treat- ment begins when acidic mine- water is filtered to remove coarse particles and then neutralised with lime, which pre- cipitates the dangerous heavy metals and neutralises the acidity. But the acid/base neutralisation reaction leaves the ‘treated’ solution contaminated with an excess of dissolved salts. Of these, the worst and most difficult to deal with is sodium in the cation form (Na + ). 63% of South Africa’s fresh water is used for irrigation in agriculture and sodium contami- nated water causes more devastation to our soils than all other dissolved substances put together. Sodium ions attach themselves to clay particles in soil and are not readily removed. Once attached, the sodium ion hydrates and causes the clay to swell making the soil impervious to water and air. Without water and oxygen penetrating the soil, agricultural yields drop quickly and drastically, a far more

serious long-term AMD associated problem than any other. South Africa’s water scarcity affects food production far more than it does the potable water shortages in urban areas, yet many of thewidely publicisedAMD successes tend to focusonpotablewater quality,which is far too expensive for agriculture to consider using. To put this argument into perspective, the daily off-take of the Rand Water Board is 5 000 M ℓ /day while all of the AMD arising across frommining activity is estimated to be about 350 M ℓ /day. To be putting a large and expensive effort into partially or completely cleaning AMD for drinking water purposes seems to bemisguided, but it must usually be promoted as a revenue source to justify the viability of a particular process. It is far more important that AMD be brought to a quality suitable for agriculture and returned to our water courses for irrigation purposes than to be trying to supplement the lesser require- ments of the city user. Of utmost importance is that any process used to clarify AMD, as well as being able to

remove the toxic heavy metals and, where applicable, residual radio-activity, is also able to substantially remove the sodiumsalts from any water being returned into our natural freshwater reservoirs. The key focus of the KNeW process is, therefore, not only todesalinatewastewaters derivedfromAMDandotherindustrialwaste- water effluents, but to go on to remove and separatethedissolvedsaltsintothosethatare valuable and the sodium salts, which are not. Counter-current ion-exchange TheKNeWprocessisawastewatertreatment process that uses ion exchange resins to re- movedissolvedcations–suchassodium(Na + ), calcium(Ca +2 )andmagnesium(Mg +2 )–andan- ions – including chloride (C ℓ - ), sulphate (SO 4 -2 ) and carbonate (CO 3 -2 ). All dissolved ions can be removed from the effluent water leaving demineralised water to any required quality. Ion exchange processes are usually oper- ated in column batch style, which is limited to solutions containing up to a maximum of 3000mg/ ℓ of dissolved solids due to the large amount of rinsewater used to regenerate the columns. In addition, there is a flow rate limit due to the highpressure drop and the process is impossible to make genuinely continuous. Thismakes these systems unsuitable for con- tinuous high volume water treatment. For treatment using the KNeW process, a twin line of stirred tank reactors (CSTRs) operate continuously, with the effluentwater flowing from tank to tank, while the resins are pumped in the opposite direction. Both volume flow and the rate of demineralisation can be varied by either increasing or decreas- ing the size or number of tanks in each side of the battery. After passing through the catex and anex ion exchange resins, the water is deminer-

For treatment using the KNeW process, a twin line of stirred tank reactors (CSTRs) operate continuously, with the effluent water flowing from tank to tank. In the foreground is a ring of PLC controlled vessels for regenerating the resins: dilute nitric acid to regenerate the cation resin; and methanol for the anion resin.

18 ¦ MechChem Africa • November 2019